Catheters are being used increasingly as a means for delivering diagnostic or therapeutic agents to internal target sites that can be accessed through the circulatory system. For example, in angiography, catheters are designed to deliver a radio-opaque agent to a target site within a blood vessel, to allow radiographic viewing of the vessel and blood flow characteristics near the release site. For the treatment of localized disease, such as solid tumors, catheters allow a therapeutic agent to be delivered to the target site at a relatively high concentration with minimum overall side effects.
Often the target site which one wishes to access by catheter is buried within a soft tissue, such as brain or liver, and can only be reached by a tortuous route (i.e., a route including repeated sharp curves) through small vessels or ducts----less than about 3 mm lumen diameter----in the tissue. The difficulty in accessing such regions is that the catheter must be quite flexible in order to follow the tortuous path into the tissue, and at the same time, stiff enough to allow the distal end of the catheter to be manipulated from an external access site, which may be as much as a meter or more from the tissue site.
Heretofore, two general methods for accessing such tortuous-path regions have been devised. The first method employs a highly flexible catheter having an inflatable, but pre-punctured balloon at its distal end. In use, the balloon is partially inflated and carried by blood flow into the target site. The balloon is continually inflated during placement to replenish fluid leaking from the balloon. A major limitation of this method is that the catheter will travel in the path of highest blood flow rate, so many target sites with low blood flow rates cannot be accessed.
In the second method, a torqueable guidewire and catheter are directed as a unit from a body access site to a tissue region containing a target site. The guidewire is bent at its distal end and may be guided, by rotating and advancing the wire, along a tortuous, small-vessel pathway, to the target site. Typically the guidewire and catheter are advanced along the tortuous pathway by alternately advancing the wire along a region of the pathway, then advancing the catheter axially over the advanced wire portion. An important advantage of this method is the ability to control the location of the catheter along a tortuous path.
It is frequently desirable, for example, in treating deep brain vessel abnormalities, to direct a small-diameter catheter along a tortuous, small-diameter pathway to the brain vessel site. The procedure may be advisable, for example, in treating an arteriovenous malformation, in order to introduce an embolic agent into the small capillaries connecting the arterial and venous vessels at a deep brain site. At a certain point along the pathway, when sharp bends are first encountered, the catheter is advanced by alternately guiding the flexibletip portion of the guidewire along the path, then threading the catheter over a portion of the advanced wire region.
One problem which may be encountered, as the guidewire and catheter are advanced, is that the guidewire can become stuck against the internal tubular surface of the catheter. Typically, this problem arises when a sharp bend, such as a hairpin loop, is encountered and/or where two or more sharp bends occur in succession. When the catheter and wire become locked together (i.e., the end of the guidewire is jammed against the internal surface of the catheter tube so as to prevent the relative movement of the guidewire and internal tubular surface) in the region of wire bending, it may be impossible to either advance or withdraw the wire. In this event, the wire and catheter must be pulled back as a unit along the pathway until both are straight enough to allow the wire to be moved axially within the catheter, and often, the physician may have to give up attempting to reach the site.
The problem of advancing a catheter over a guidewire in a region of sharp wire bend(s) has been addressed by the catheter construction disclosed in U.S. Pat. No. 4,739,768. This construction includes a relatively long, relatively rigid proximal segment, and a shorter, more flexible distal segment having a length of at least about 5 cm. The proximal segment provides sufficient torqueability and axial stiffness for guiding the catheter and internal guidewire from a body access site to the target tissue of interest. Once the tortuous tissue pathway is reached, the more flexible end segment allows the end region of the catheter to be advanced axially over sharp and/or frequent wire bends.